A falling stone takes 0.28 s to travel past a window 2.2 m...

[teatime95]

Homework Statement

A falling stone takes 0.28 s to travel past a window 2.2 m tall.
From what height above the top of the window did the stone fall?
So from this problem I think I have these variables:
vo = ? (or 0 because falling usually indicates 0 for an initial velocity?)
yo = 0
y = 2.2m
t = 0.28s
a = -9.8m/s^2

Homework Equations

Kinematic equations:

y=yo+vo*t + 1/2*a*t^2
v^2 = v^20 + 2a(y-y0)

The Attempt at a Solution

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I originally tried to use the first equation above, and solve for vo if I assumed it was not equal to zero.
So I isolated vo and got,
vo = -2.2 + 1/2(-9.8m/s^2)(0.28s)^2 / -0.28s
vo = 9.229m/s

After which I tried to substitute that value in my second equation which also didn't give me the correct answer for height (which i think is the new yo)
My textbook gives me the final answer but not the steps that it reached to get to it.

I have included the final answer here in white ink (so highlight to read) In case anyone wants to check their work or work backwards: [COLOR=#black]2.1m [/COLOR]

I feel like my process and identifying my variables given correctly is wrong and I'm at that point where I am confusing myself repeating this question.
Guidance would greatly be appreciated,
Thank you all!
Newbie in Physics

 

[Doc Al]

I originally tried to use the first equation above, and solve for vo if I assumed it was not equal to zero.
So I isolated vo and got,
vo = -2.2 + 1/2(-9.8m/s^2)(0.28s)^2 / -0.28s

That's an OK method. But redo it more carefully.

yo = 0
y = 2.2m

Since the stone is falling, that final value of y must be negative.

 

[teatime95]

That's an OK method. But redo it more carefully.

Is my math wrong here^ am I isolating for the correct variable?

Since the stone is falling, that final value of y must be negative.

If I change 2.2 to -ve I still get the same number -9.229, and I am still unsure what I do after?

 

[Doc Al]

If I change 2.2 to -ve I still get the same number -9.229, and I am still unsure what I do after?

I get a different value for v0.

Show what you've put into this equation:

y=yo+vo*t + 1/2*a*t^2

Don't isolate v0 yet, just show what you have for the values in that equation.

 

[J Hann]

Check your signs.
If downward is negative then both V0 and g have negative signs, they both end up with negative signs in your final equation for V0.